These studies have focused on: 1) size at birth as measured by head circumference, crown-heel length, and birth weight in relation to gestational age and perinatal mortality, 2) the tendency to repeat similar birth weight and gestational age in subsequent pregnancy outcomes to the same mothers, 3) perinatal mortality in relation to order of birth and size of sibship, 4) epidemiologic risk factors for preterm birth, 5) epidemiologic risk factors for small-for-gestational age births, 6) contribution of multiple births to perinatal mortality rates.